This invention relates to the analysis of gas mixtures and refers more specifically to gas analyzers for determining the concentration of a constituent in a mixture of gases, e.g. automobile exhaust gases or the like. The enactment of federal and state legislation to control automobile emissions has resulted in a requirement to measure automobile emissions outside of the laboratory. Several portable automobile emissions analyzers have appeared in the last few years, all claiming to fill the need for quick, easy accurate measurement of exhaust gaseous concentrations. Almost without exception, the measurement techniques employed in these portable analyzers are identical with those used in their more costly and more complex laboratory counterparts. However, the compromises required to employ these techniques in low cost, portable packages combined with the lack of skill of their intended operators and the hostility of their intended environment has resulted in a generation of instruments which are far from ideal.
The present invention provides a portable gas analyzer which quickly, easily and accurately determines the concentration of one or more gaseous components of a mixture of gases. A gas analyzer of the present invention requires no span gases for calibration; the instrument is calibrated by a zeroing procedure which utilizes ordinary air. The background level of radiation is determined and is used to appropriately compensate the output signals of the analyzer so that true indications of the concentration of gaseous components under investigation are obtained.
In a preferred single path embodiment, the analyzer includes a source of radiant energy, preferably infrared energy; a detector for the radiant energy; means for interposing the gas mixture between the source and the detector, for example, a sample cell; means for sequentially interposing a reference filter, a filter for each gaseous component under investigation, and a source blocking device between the source and the detector; means for compensating the output signal of the detector in accordance with the background level of radiation as determined when the source blocking device is interposed between the source and the detector; and means for compensating the gaseous components under investigation for absorption band interferences. With the above signal processing system, an output signal may be provided which may be displayed substantially in real time.
By way of example, the source blocking means, the filter for each gaseous component under investigation, and the reference filter may be mounted on a wheel which is positioned between the sample cell and the detector, preferably in close proximity to the detector so that the filter wheel and components constitute substantially the only source of background radiation whereby the background contribution to the signals is closely controlled and accurately determined. However, each of the above may be positioned in any suitable location intermediate the source and the detector. Also, by way of example, the background signal obtained when the source blocking means is interposed between the source and the detector may be stored for sequential subtraction from each output signal corresponding to the respective gaseous components under investigation as well as from a reference signal. By measuring each of the component, reference, and background signals independently of all previous measurements of the component, reference, and background signals on each cycle and subtracting the background signal from at least one of the signals on each cycle, the DC component of noise and a substantial portion of the low frequency component of noise is removed on each cycle. Consequently, there are no systematic errors since the means noise level is zero. That is to say, if the output signal is time averaged over a sufficiently long time, an error free signal can be obtained.
Compensation for absorption band interferences may be accomplished in a similar manner by storing and sequentially utilizing a signal representative of each interfering gaseous component. In the preferred form, each of the signals corresponding to the gaseous components under investigation are normalized by providing a ratio of the gaseous component signals and the reference signal, the reference signal having been obtained when a filter having a spectral bandpass at which the gaseous components under investigation have negligable absorption is interposed between the source and the detector. This is advantageously accomplished using an analog divider incorporating a photocell and an associated radiant source. More particularly, the photocell is disposed in a feedback circuit of an operational amplifier to adjust the circuit gain in accordance with the resistance of the photocell. A light source is used in association with the photocell which is intensity-controlled by the reference signal level appearing at the output of the operational amplifier to correspondingly affect the gain of the operational amplifier by changing the resistance of the photocell. In effect, a highly effective, yet simple, analog divider is provided.
The above described processing circuitry for the single path analyzer may be also used with a dual path design with slight modification to accommodate the derivation of a reference signal through the use of a separate cell containing a reference gas rather than the aforementioned reference filter.